[docsyserver] Turn on mermaid support.
[docsyserver] Turn on mermaid support.

Bug: skia:11799
Change-Id: Ic3ecdb31b0833285a72c1d9f8be03a89eb8be7ac
No-Try: true
Docs-Preview: https://skia.org/?cl=391821
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/391821
Reviewed-by: Joe Gregorio <b9cf2471ff6d33504cafe8e6f356b732b2cadced@google.com>

aosp-mirror/platform_external_skia,aosp-mirror/platform_external_skia,google/skia,aosp-mirror/platform_external_skia,google/skia,aosp-mirror/platform_external_skia,google/skia,google/skia,aosp-mirror/platform_external_skia,aosp-mirror/platform_external_skia,google/skia,aosp-mirror/platform_external_skia,aosp-mirror/platform_external_skia,google/skia,aosp-mirror/platform_external_skia,google/skia,google/skia,google/skia,aosp-mirror/platform_external_skia,google/skia